PLECS Blockset uses the Simulink solver. If you are using one of Simulink’s fixed-step solvers then the entire model is solved at the same time step.
Generally the best approach would be to use a variable step solver to model any physical system (PLECS part) and to place any discrete controls inside of an atomic subsystem. Then the discrete controls will execute at the fixed rate you specify, while the variable step solver will handle the continous system (electric circuit).
See the Mathwork’s write-up on Sample Time in Systems. You can assign the sample time for an atomic subsystem by right clicking the block and opening the Block Parameters window.
If you used a fixed step solver, the overall approach is the same to specify the controls to run at a slower rate than the solver’s base time step.